The invention relates to a series-wound motor, in particular a universal motor for an electric tool, having an armature with a commutating coil, which is rotatably mounted in a stator that forms a closed yoke and which is supplied with power by brushes. The stator comprises at least two field poles, each having a center section being integral with the closed yoke. From each center section two pole horns extend circumferentially defining axial grooves extending between the closed yoke and the pole horns.
Series-wound motors may be operated, selectively, with a first direction of rotation or with a second direction of rotation and may also comprise a switch for reversing the direction of rotation.
Alternatively, series-wound motors may have a preferred direction of rotation. In this case each pole comprises a pole horn with a run-on edge and a pole horn with a run-off edge. Also such series-wound motors having a preferred direction of rotation may have a braking system for active, self-exerted braking or for braking initiated in a different way.
In the retarded series-wound motors usually at least one field coil and a switching means are provided for switching between motor operation and brake operation. In motor operation, the field coil is connected in series with the armature coil in a motor circuit supplied with a source voltage. In brake operation, the field coil forms a closed braking circuit with the armature coil, separated from the voltage source.
A series-wound motor of this type is disclosed in German patent DE 196 36 519. The known motor is a universal motor with a pole package having a field pole arrangement of two pole portions each, where the pole package is designed for a predetermined rotary direction. The motor has a switching arrangement between motor and brake operation and includes a current path parallel to the field coil containing a diode array. To achieve good commutation both in motor operation and in brake operation, an additional field coil is provided which surrounds the field coil at the run-off edge of the two pole horns with a special configuration of the pole plates and is separately located at the run-on edges of the pole horns.
To ensure a sufficient commutation in motor operation, the commutation of the armature coil in universal motors is normally displaced with respect to the geometric neutral zone counter to the running direction. This normally is achieved in that the carbon brushes are shifted counter to the rotational direction of the armature out of the neutral zone. In this manner, reduced sparking is achieved, without commutator windings being necessary. If such a universal motor is to be retarded or braked by reversing the poles of the armature coil or the field coil with a switching device and by short circuiting the motor, then a deficient commutation results during the braking phase of the motor, if the brushes are not adjusted or no commutating poles are provided.
This problem is avoided in the above-mentioned motor through the use of additional coils in conjunction with the special winding arrangement, however in comparison to conventional universal motors which only require two field coils (a subdivided field coil) and a displacement of the brushes out of the geometric neutral zone counter to the running direction of the motor, the construction of the above motor is considerably more complicated. Furthermore, a much greater weight results for the same motor performance or, for the same weight, a reduced performance.
The use of commutating windings or additional coils to avoid the mentioned commutation problems is considered to be a drawback because such motors are used particularly for electric tools, in which a high performance with the smallest possible weight is important and in which large volumes of motors are to be produced at the most inexpensive cost.
Also series-wound motors not having a preferred direction of rotation always may have particular problems with sparking which requires a regular service for replacing the brushes after certain operating times. Consequently, there exists a continuous need for a series-wound motor having reduced sparking.
It is a first object of the present invention to provide an improved series-wound motor, which guarantees a sufficiently good commutation and in which excessive sparking is avoided.
It is a second object of the present invention to disclose an improved series-wound motor, having increased power without increasing outer dimensions.
It is a third object of the invention to disclose an improved series-wound retarded motor, which guarantees a sufficiently good commutation and in which excessive sparking in particular during a braking operation is avoided.
It is a further object of the invention to provide an improved series wound motor which allows active braking without the need for supplementary windings.
It is a further object of the invention to provide an improved series wound motor that allows self-excited braking and is very reliable.
These and other objects of the present invention are achieved in a series-wound motor of the above-mentioned type by providing at least one pole horn with first and second protrusions protruding circumferentially from an axial edge of the pole horn. In a different embodiment each pole horn may be provided with first and second protrusions protruding circumferentially from an axial edge of the pole horn.
The object of the invention may be achieved in a retarded series-wound motor by making the pole horns at the run-off side shorter than the pole horns at the run-on side.
Alternatively, at least on the run-off side the poles comprise at least two protrusions extending in circumferential direction. The protrusions that may, preferably, be located at the axial ends of the stator, define a cutout section therebetween. Also on the run-on side such protrusions may be provided.
The inventor has found that by providing such protrusions the neutral zone is extended. The invention provides for more room for attaching a field coil to the field pole. This is important, in particular, with respect to the utilization of preformed coils (coils that have been wound externally and that are usually stabilized by wrapping with tape). The preformed coils are then attached to the field poles by inserting them into the axial grooves formed between the closed yoke and the pole horns extending therefrom. The protrusions facilitate broader field coils, since the winding package is held by the protrusions extending from the axial edge of the respective pole horn.
This leads to a dramatic decrease in sparking. It was found that service life of the brushes was increased almost threefold when compared with conventional arrangements. Also power of the motor may be increased without increasing outer dimensions of the motor.
The reduction in sparking is considerably important for motors that allow switching between two directions of rotation, as well as for motors having a preferred direction of rotation and a braking system.
While unbraked series-wound motors with a preferred direction of rotation may have a pole overlap of roughly 120 to 125 degrees, smaller pole overlaps of roughly 105xc2x0 to 110xc2x0 are made possible by the invention for motors that have two directions of rotation as well as for retarded motors.
Also it has been found in retarded motors with braking systems that no negative influence occurs during the normal motor operation mode due to the shortening of the pole horns at the run-off edge, or due to the arrangement of recesses at the run-off edge of the pole horns, while at the same time an increased commutating sparking is avoided in brake operation. The commutating armature coil can be displaced counter to the running direction from the geometric neutral zone, for which purpose the brushes are preferably arranged to be rotated counter to the rotary direction of the armature out of the neutral zone. Basically, however, it is also possible to configure the circuit switching connections so that a brush displacement is reached.
Commutating windings or other additional coils can be relinquished with such an arrangement of the field poles at the run-off edges of the pole horns, because a distinctly improved commutation is achieved in this manner also in brake operation. In brake operation, an enhanced concentration of the magnetic field lines arises at the run-off side of the pole horn, while in motor operation this takes place at the run-on sides of the pole horns. The increased brush sparking in motor operation mode caused by unfavorable arrangement of the brushes is counteracted by the shortening of the pole horns on the runoff side or by the use of recesses or cutouts.
At the same time, a distinctly improved self-excitation results for self-exciting retardation, so that a reliable braking of the motor occurs when switching to the braking mode.
In a preferred embodiment of the present invention, at least two protrusions or tongues are provided extending in circumferential direction on the respective run-on edges of the pole horns, between which the at least one recess is formed. In this manner, an impairment of the motor behavior in the motor operation phase can be practically completely avoided, while at the same time, the desired improvements in the braking phase are achieved. In addition, a good placement of the field coil winding is achieved on the run-off edge of the pole horn.
As mentioned, the armature coil is preferably displaced with respect to the geometric neutral zone contrary to the preferred rotary direction.
In addition, means are preferably provided to limit the brake current flowing in brake operation. Two anti-parallel diode arrays can be employed in known manner, which are switched to be parallel to the field coil in brake operation.
According to another embodiment of the invention, a transformer is provided connected to the power grid, whose secondary winding is connected parallel to the field coil in the brake circuit, where a control switch, preferably a transistor is provided to control the current flowing in the brake circuit across the armature coil and the field coil. Preferably, the control switch is a field effect transistor, which is connected with its source and drain to be parallel with the field coil and which controls the current through the field coil depending on the current flowing in the armature coil.
With this configuration, a current is introduced into the brake circuit through the secondary winding of the transformer, which ensures a reliable initiation of braking in all circumstances. In this manner, a reliable initiation of the braking by switching to the braking mode is ensured even in the most unfavorable situations. Through the field effect transistor, it is possible to regulate the brake current even in the advanced stage of the braking process such that a strong braking moment is present. The braking characteristic is greatly improved to ensure a short braking time. The braking characteristic can be adapted such that a slow running out of the motor at the end of braking can be avoided. For this purpose, the field effect transistor is preferably connected with its gate through a voltage divider to the brushes and thus also to the armature coil.
Preferably, a load resistor is provided in the brake circuit, which is connected through a diode to one brush and one end of the field coil via a diode. One end of the load resistor is connected to the drain of the field effect transistor. The source of the field effect transistor is connected to the other brush and the other end of the field coil.
According to another preferred embodiment of the invention the secondary winding of the transformer is coupled via a rectifier circuit, preferably via a bridge rectifier in parallel to the field winding, wherein the positive output of the bridge rectifier is coupled to drain and the negative output is coupled to source of the field effect transistor.
It will be understood that the above-mentioned features and those to be discussed below are applicable not only in the given combinations but may be used in other combinations or taken alone without departing from the scope of the invention.